8.14. Channel Mixer

8.14.1. Overview

This command combines values of the RGB channels. It works with images
with or without an alpha channel. It has monochrome mode and a preview.

8.14.2. Activate the command

You can find this command through
Colors → Components → Channel Mixer.

8.14.3. Options

Figuur 16.149. “Channel Mixer” command options

Output Channel

From this menu you select the channel to mix to. Choices are
Red, Green, or Blue. It is insensitive when
Monochrome option is checked.

Red, Green, Blue

These three sliders set the contribution of red, green or blue
channel to output. Can be negative. These sliders are graduated
from -200 to 200. They represent the percentage which will be
attributed to the output channel. 100% corresponds to the value of
the channel of the studied pixel in the image.

Monochrome

This option converts the RGB image into a gray-scale RGB image.
The Channel Mixer command is often used with this aim in view,
because it often provides a better result than the other ways
(see Grayscale in
Glossary). Makes the Output Channel menu
insensitive.

Opmerking

The 21%, 72%, 7% settings give you the same gray luminosity
(Value) as the Grayscale command in Image/Mode. (They were 30%,
59%, 11% in v2.2).

Preserve Luminosity

Calculations may result in too high values and an image too much
clear. This option lessens luminosities of the color channels
while keeping a good visual ratio between them. So, you can change
the relative weight of the colors without changing the overall
luminosity.

8.14.4. Buttons

Open

Load settings from a file.

Save

Save settings to a file.

Reset

Set default settings.

8.14.5. How does Channel Mixer work?

In RGB mode

In this mode, you have to select an Output Channel
. This channel is the one which will be modified. In
the dialog window, its default value is 100%, corresponding to
the value of the channel in the original image. It can be
increased or decreased. That's why slider ends are -200 and 200.

Three RGB sliders let you give a percentage to every channel. For
every pixel in the image, the sum of the calculated values for
every channel from these percentages will be given to the Output
Channel. Here is an example:

Figuur 16.150. The original image and its channels

RGB values of the pixels in red, green, blue, gray squares
are displayed. The black rectangle is special, because black
(0;0;0) is not concerned by the command (0 multiplied by any
percentage always gives 0). The result can't exceed 255 nor
be negative.

Figuur 16.151. Output channel is red. Green Channel +50

In the red square, the pixel values are 230;10;10. Relative
values are 1;0.5;0. The calculation result is 230*1 + 10*0,5
+ 10*0 =235. The same reasoning is valid for the green and
the blue squares.

In the gray square, which contains red color, the
calculation result is above 255. It is reduced to 255. A
negative value would be reduced to 0.

In every square, pixels have been converted into a gray
level equal to the value of the Red channel in the original
image (The background has been painted with pink afterwards
to make all squares visible).

Here is how the Preserve Luminosity
works in the monochrome mode:
“
For example, suppose the sliders were Red:75%, Green:75%,
Blue:0%. With Monochrome on and the
Preserve Luminosity
option off, the resulting picture would be at 75%+75%+0% =150%,
very bright indeed. A pixel with a value of, say,
R,G,B=127,100,80 would map to 127*0.75+100*0.75+80*0=170 for
each channel. With the Preserve Luminosity option on, the
sliders will be scaled so they always add up to 100%. In this
example, that scale value is 1/(75%+75%+0%) or 0.667. So the
pixel values would be about 113. The Preserve Luminosity option
just assures that the scale values from the sliders always adds
up to 100%. Of course, strange things happen when any of the
sliders have large negative values
”
(from the plug-in author himself).

Opmerking

Which channel will you modify?
This depends on what you want to do. In principle, the Red
channel suits contrast modifications well. The Green channel is
well adapted to details changes and the Blue channel to noise,
grain changes. You can use the
Decompose
command.